1. Field of the Invention
This invention relates to an apparatus for entraining and mixing powdered solids into a liquid stream with an eductor, and a control system therefor.
2. Description of the Prior Art
Devices known as aspirators, eductors or ejectors are commonly used to provide inline mixing of two flowing streams. A pressurized primary motive fluid is passed through a nozzle at high velocity. Its momentum and kinetic energy are used to entrain, mix and compress a second fluid stream. The primary pressurized fluid and secondary (entrained) fluid are typically both liquid or both gaseous. In the latter case, the device is known as a pneumatic mixing tube or pneumatic conveyor.
One such system using liquid streams is shown in Dragon U.S. Pat. No. 2,683,622, where a fertilizer solution is mixed with diluting water in a "Venturi section" comprising an eductor.
In another system disclosed by Reid et al U.S. Pat. No. 3,607,105, dry chemical powders are mixed and dissolved with water to make lump-free solutions, using an aspirator.
In Ellis U.S. Pat. No. 4,085,043, an aqueous suspension of granular carbon is moved from vessel to vessel in a carbon contact system, using pressurized water passing through eductors.
Special problems often occur when one of the streams contains solids particles. For example, in Wilson U.S. Pat. No. 2,920,635, a method for intermittently transferring a slurry from a vessel into an aspirating stream of water is shown. Plugging of the transfer line between the slurry vessel and a flow control valve is a problem which his invention addresses by applying a continuously flowing backwash stream.
Transfer of a dry powder such as powdered activated carbon (PAC) into a liquid stream such as water introduces further problems. When wetted in the range of about 40 to 90 percent water, PAC assumes the consistency of a sticky paste which clings to the surfaces of the hopper. Therefore it is important to (a) remove all solids from the hopper and conduit at the end of each flow cycle and (b) ensure that the hopper and conduit are essentially free of liquid before initiating the flow of powdered solids.
Periods of low process-water pressure are a common occurence in many industrial plants and wastewater treatment installations. Where such water is used as the primary fluid in an eductor system, the energy entraining the solids may be insufficient to handle all of the powdered solids entering the eductor and the system may become clogged, particularly where the solids enter the eductor. The water pressure may drop to a level where suction is lost and water floods the pipe carrying the powdered solids. Plugging of the slurry pipeline downstream from the eductor will have the same result, that is, a paste of powdered solids and water at the entrance to the eductor will prevent movement of dry powdered solids into the eductor.